Remediation of metal contaminants with hydrocarbon-utilizing bacteria

ABSTRACT

Methods and apparatus are disclosed for remediating metal contaminants using hydrocarbons which stimulate the growth of hydrocarbon-utilizing bacteria. The metal contaminants may include heavy metals such as arsenic, antimony, beryllium, cadmium, chromium, copper, lead, mercury, iron, manganese, magnesium, radium, nickel, selenium, silver, thallium and zinc. The hydrocarbon may include alkanes, alkenes, alkynes, poly(alkene)s, poly(alkyne)s, aromatic hydrocarbons, aromatic hydrocarbon polymers and aliphatic hydrocarbons. Butane is a particularly suitable hydrocarbon which stimulates the growth of butane-utilizing bacteria. Remediation may occur in-situ or ex-situ, and may occur under aerobic, anaerobic or dual aerobic/anaerobic conditions. Examples of applications include the remediation of heavy metals, the remediation of arsenic impacted surface water, groundwater and/or soil, the remediation of acid mine drainage, and the treatment of spent metal plating solutions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/205,816 filed Jul. 26, 2002, which is a continuation-in-part of U.S.patent application Ser. No. 09/878,656 filed Jun. 11, 2001, which is acontinuation of U.S. patent application Ser. No. 09/293,088, now U.S.Pat. No. 6,244,346, which is a continuation-in-part of U.S. patentapplication Ser. No. 09/275,320, now U.S. Pat. No. 6,245,235, which is acontinuation-in-part of U.S. patent application Ser. No. 08/767,750, nowU.S. Pat. No. 5,888,396, which are incorporated herein by reference.Application Ser. No. 10/205,816 also claims the benefit of U.S.Provisional Application Ser. Nos. 60/308,487, 60/308,210 and 60/308,212filed Jul. 27, 2001, and U.S. Provisional Application Ser. No.60/344,868 filed Dec. 31, 2001, which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to the remediation of metal contaminants,and more particularly relates to the remediation of metal contaminantswith hydrocarbon-utilizing bacteria.

BACKGROUND INFORMATION

Various types of metal contaminants are present in surface water,groundwater, soil, storage tanks, lagoons, industrial gaseous emissionsand other sites, often as wastes or byproducts of industrial processes.Arsenic, antimony, beryllium, cadmium, chromium, copper, lead, mercury,iron, manganese, magnesium, radium, nickel, selenium, silver, thalliumand zinc are considered to be priority pollutants by the U.S.Environmental Protection Agency (EPA).

Arsenic contamination in surface water, groundwater and soil representsa significant health hazard. Arsenic is used for hardening metals suchas copper and lead and as a doping agent in the electronics industry.Arsenic salts are used to make herbicides, rodenticides and fireworks.Arsenic and arsenic compounds are toxic and can be carcinogenic. Theyare absorbed into the body through gastrointestinal ingestion orinhalation. For example, the trivalent inorganic compounds of arsenic,such as arsenic trichloride, arsenic trioxide and arsine, are highlytoxic.

Arsenic-contaminated groundwater has conventionally been treated bygroundwater pump and treat technologies including post precipitation,chemical oxidation, filtration, sedimentation, etc. With respect to thetreatment of soils contaminated with arsenic, the following methods arecurrently employed: 1) land farming, where soil piles are watered andaerated; 2) bioreactors that involve the slurry treatment of soil andwater in a closed vessel to which oxygen, nutrients and a carbohydratecosubstrate such as molasses, corn syrup, or hydrolyzed starch areadded; and 3) in-situ treatment where contaminated soils are chemicallyoxidized and/or stabilized using encasement methods.

Contamination of groundwater and surface water by acid mine drainage(AMD) and heavy metals is also a global problem. Disposal of materialssuch as mine tailing, waste rock and spent oil shale have created severeenvironmental problems. AMD is contaminated effluent that results fromthe oxidation of iron-sulfide minerals exposed to air and water. AMD isgenerated by chemical reactions and bacterial oxidation processes.Sulfide ores contain large quantities of pyrite, which is discarded inthe tailings and produces sulfuric acid when exposed to water andoxygen. The ferrous iron produced is then oxidized to ferric ions, whichbecome the dominant oxidizing agent of the exposed sulfide minerals. Thereduced sulfur and iron compounds in the deposit provide an environmentfor T. ferrooxidans which oxidize iron, thiosulfate, sulfur and metallicsulfides to obtain energy for growth while using oxygen as the finalelectron acceptor and CO₂ as its sole source of carbon. This processgenerates an acidic pH.

AMD resulting from all types of metal mining operations is one of themost pressing environmental problems facing the mining and mineralindustries. A significant portion of the AMD draining into rivers andstreams is released from waste rock. Once the AMD process has begun, itis extremely difficult to reverse or stop.

Conventional remediation options for groundwater impacted by AMD includepreventing the infiltration of contaminants, stabilizing thecontaminants chemically, or removal and treatment of the contaminatedgroundwater. In addition, subaqueous disposal of mine tailings has beenemployed to avoid terrestrial AMD. However, severe environmental impactsresult from subaqueous tailings disposal, including increased turbidityin the receiving waters, sedimentation, toxicity, contamination and fishkills.

Electrolytic plating solutions normally contain high concentrations ofheavy metals like zinc, chromium, cadmium, nickel, selenium, copper,gold, silver and nickel. Electroless nickel plating solutions contain anickel metal salt, such as sulfate, acetate, carbonate or chloride salt,pH adjustors, accelerators, stabilizers, buffers, and wetting agents.The electroless nickel solutions only have a limited useful life andeventually become depleted or spent. The disposal or treatment of spentelectrolytic metal plating solutions poses significant challenges forthe electroplating industry. The dissolved metal concentration must bebelow discharge thresholds in order to allow for the solution to bedischarged as non-toxic waste directly to a municipal wastewatertreatment facility. The spent solutions from the electrolytic andelectroless plating processes pose a severe hazard to the environment,if disposed of improperly, and a high monetary cost, if disposed ofproperly.

A number of wastewater treatment processes have been developed to reducethe metal content in spent plating solutions to low levels prior todischarge. Many current methods involve the removal of dissolved metalfrom solution by chemical reduction. The spent electroless solution isfirst contacted with a reducing agent for sufficient time to cause thedissolved metal salt to undergo chemical reduction, resulting in theprecipitation of the metal compounds out of the solution. Some methodsinclude the dosing of electroless baths with caustic soda to precipitatethe bulk of the heavy metal contaminants as insoluble hydrous oxides(metal hydroxides), pressing the sludge into a filter cake, drumming anddisposal. Another waste treatment used for spent electroless platingsolutions is the dosing of the solution at slightly alkaline pH withreducing agents. The reducing agents typically used to convert thedissolved metal salt into insoluble metal precipitates include sodiumborohydride, sodium hydrosulfite and other chemicals. A further wastetreatment method known for reducing the dissolved metal content of spentelectroless baths to acceptable discharge levels involves organosulfurprecipitation of the metal by dosing the spent solution at a pH of 5-8with water-soluble precipitating agents.

The bioremediation of various pollutants with butane-utilizing bacteriais disclosed in U.S. Pat. Nos. 5,888,396, 6,051,130, 6,110,372,6,156,203, 6,210,579,6,245,235 and 6,244,346, each of which isincorporated herein by reference.

SUMMARY OF THE INVENTION

In accordance with the present invention, hydrocarbon-utilizing bacteriaare used to remediate metal contaminants. The bacteria use hydrocarbonsas a substrate under aerobic, anaerobic or dual aerobic/anaerobicconditions. In a preferred embodiment, the hydrocarbon comprises atleast one alkane such as butane, methane, ethane and/or propane.Examples of applications include the remediation of heavy metals, theremediation of arsenic impacted surface water, groundwater and/or soil,the remediation of acid mine drainage, and the treatment of spent metalplating solutions.

An aspect of the present invention is to provide a method of remediatinga metal contaminant. The method includes treating the metal contaminantwith hydrocarbon-utilizing bacteria in the presence of a hydrocarbon.

Another aspect of the present invention is to provide a method oftreating a metal-contaminated site. The method includes supplying ahydrocarbon substrate to the site to thereby remediate the metalcontaminant.

A further aspect of the present invention is to provide a system forremediating a metal contaminant. The system includes means for treatingthe metal contaminant with hydrocarbon-utilizing bacteria in thepresence of at least one hydrocarbon.

Another aspect of the present invention is to provide a remediationsystem for treating a metal contaminant comprising a source ofhydrocarbon substrate and at least one injector in flow communicationwith the hydrocarbon substrate and the metal contaminant.

These and other aspects of the present invention will be more apparentfrom the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of an in-situ hydrocarbon injectionsystem for remediating metal-contaminated groundwater in accordance withan embodiment of the present invention.

FIG. 2 is a schematic side view illustrating an ex-situ treatment systemfor metal-contaminated soil in accordance with an embodiment of thepresent invention.

FIG. 3 is a schematic plan view of an in-situ hydrocarbon injectionsystem for remediating acid mine drainage in accordance with anembodiment of the present invention.

FIG. 4 is a schematic side view of an ex-situ treatment system employinghydrocarbon injection within a precipitation lagoon for the remediationof acid mine drainage in accordance with an embodiment of the presentinvention.

FIG. 5 is a schematic side view of an ex-situ treatment system for theremediation of spent metal plating solutions in accordance with anembodiment of the present invention.

FIG. 6 illustrates an anaerobic bioreactor in accordance with anembodiment of the present invention.

FIG. 7 illustrates an aerobic bioreactor in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

The present invention uses hydrocarbon-utilizing bacteria to remediatemetal contaminants. The metal contaminants may include one or moremetals listed on the Periodic Table, such as arsenic, antimony,beryllium, cadmium, chromium, copper, lead, mercury, iron, manganese,magnesium, radium, nickel, selenium, silver, thallium and zinc, as wellas compounds containing such metals. In one embodiment of the presentinvention, the concentrations of metal contaminants in groundwater arereduced to the EPA Maximum Contaminant Levels (MCLs) set forth in theNational Primary Drinking Water Regulations shown in Table 1 below.These MCLs represent safe levels for drinking water that metalconcentrations in groundwater should not exceed. In the case of arsenic,the EPA has recently lowered the MCL from 0.05 milligrams per liter(mg/l) to 0.01 mg/l due to the toxic and carcinogenic nature of thismetal. TABLE 1 EPA Maximum Contaminant Levels Metal Contaminant MCL(mg/l) Arsenic  0.01 Antimony 0.006 Beryllium 0.004 Cadmium 0.005Chromium  0.1 Copper  1.3 (Action Level) Lead 0.015 (Action Level)Mercury 0.002 Iron  0.3 (Secondary MCL) Manganese  0.05 (Secondary MCL)Selenium  0.05 Silver  0.10 (Secondary MCL) Thallium 0.002 Zinc    5(Secondary MCL)

Metal contaminants may be present in various media, for example, insoil, groundwater, surface water, storage tanks, lagoons, industrialgaseous emissions, waste rock, soil piles, agricultural soils andfertilizers, aquatic systems, paints, polymers, herbicides, pesticidesand spent auto fluids such as antifreeze and waste oil. Some examples ofmetal contaminants include acid mine drainage, metal plating solutions,arsenic-impacted groundwater, metallic salts used to control algae inlakes, weed control chemicals for lawns, pollution in river sedimentsand lakes, urban highway runoff, metal surface treatment waste, metalcutting and fabrication dust and junked auto parts. Metal contaminantsmay be present at many different industrial sites such as miningfacilities, smelting operations, foundries, steel mills, metalprocessing and manufacturing facilities, process plants, productionfacilities for computer chips and semiconductors, and the like. Sometypes of metal contaminants may be radioactive, such as nuclear waste,armor plating production waste, munitions and ordnance, and hospitalwaste.

The present invention uses hydrocarbon-utilizing bacteria in thepresence of at least one hydrocarbon substrate to remediate metalcontaminants. The hydrocarbon may comprise one or more alkane, alkene,alkyne, poly(alkene), poly(alkyne), aromatic hydrocarbon, aromatichydrocarbon polymer, or aliphatic hydrocarbon. In a preferredembodiment, the hydrocarbon comprises at least one alkane such asbutane, methane, ethane and/or propane. For example, butane is anontoxic and relatively low molecular weight organic compound that mayserve as an electron donor under aerobic or anaerobic conditions. Thehigh solubility of butane provides a large zone of influence and makesbutane particularly suited to accelerate the transformation of aerobicconditions to anaerobic conditions. Under aerobic conditions, butanesubstrates stimulate the growth of butane-utilizing bacteria which mayoxidize iron and other metals.

In a preferred embodiment, butane is the most prevalent compound of thehydrocarbon substrate on a weight percent basis, and typically comprisesat least about 10 weight percent of the hydrocarbon substrate. The otherconstituents of the hydrocarbon substrate may include other alkanes orother types of hydrocarbons, and may also include inert gases. Thehydrocarbon substrate preferably comprises at least about 50 weightpercent butane. More preferably, the hydrocarbon substrate comprises atleast about 90 weight percent butane. In a particular embodiment, thehydrocarbon substrate comprises at least about 99 weight percentn-butane. The butane may contain straight (n-butane) and/or branchedchained compounds such as iso-butane.

Hydrocarbon-utilizing microorganisms used in accordance with the presentinvention are typically found naturally in the affected media. However,in some applications, it may be necessary to inoculate bacteria into thetreatment zone. Suitable bacteria may include the following Groups (inaddition to fungi, algae, protozoa, rotifers and all other aerobic andanaerobic microbial populations found in decaying materials):

-   -   Group 1: The Spirochetes    -   Group 2: Aerobic/Microaerophilic, motile, helical/vibroid,        gram-negative bacteria    -   Group 3: Nonmotile (or rarely motile), gram-negative bacteria    -   Group 4: Gram-negative aerobic/microaerophilic rods and cocci    -   Group 5: Facultatively anaerobic gram-negative rods    -   Group 6: Gram-negative, anaerobic, straight, curved, and helical        bacteria    -   Group 7: Dissimilatory sulfate- or sulfur-reducing bacteria    -   Group 8: Anaerobic gram-negative cocci    -   Group 10: Anoxygenic phototrophic bacteria    -   Group 11: Oxygenic phototrophic bacteria    -   Group 12: Aerobic chemolithotrophic bacteria and associated        organisms    -   Group 13: Budding and/or appendaged bacteria    -   Group 14: Sheathed bacteria    -   Group 15: Nonphotosynthetic, nonfruiting gliding bacteria    -   Group 16: The fruiting, gliding bacteria and the Myxobacteria    -   Group 17: Gram-positive cocci    -   Group 18: Endospore-forming gram-positive rods and cocci    -   Group 19: Regular, nonsporing, gram-positive rods    -   Group 20: Irregular, nonsporing, gram-positive rods    -   Group 21: The mycobacteria    -   Groups 22-29: The actinomycetes    -   Group 22: Nocardioform actinomycetes    -   Group 23: Genera with multiocular sporangia    -   Group 24: Actinoplanetes    -   Group 25: Streptomycetes and related genera    -   Group 26: Maduromycetes    -   Group 27: Thermomonospora and related genera    -   Group 28: Thermoactinomycetes    -   Group 29: Genus Glycomyces, Genus Kitasatospira and Genus        Saccharothrix    -   Group 30: The Mycoplasmas—cell wall-less bacteria    -   Group 31: The Methanogens    -   Group 32: Archaeal sulfate reducers    -   Group 33: Extremely halophilic, archaeobacteria (halobacteria)    -   Group 34: Cell wall-less archaeobacteria    -   Group 35: Extremely thermophilic and hyperthermophilic        S⁰-metabolizers.

In addition, suitable bacteria may include facultative and/ormicroaerophilic anaerobes, which are capable of surviving at low levelsof oxygen. These bacteria do not require strict anaerobic conditionssuch as the obligate anaerobes. Acidophilic, alkaliphilic, anaerobe,anoxygenic, autotrophic, chemolithotrophic, chemoorganotroph,chemotroph, halophilic, methanogenic, neutrophilic, phototroph,saprophytic, thermoacidophilic and thermophilic bacteria may be used.Hydrocarbon and oxygen injection may encourage the growth of othermicroorganisms such as fungi, protozoa and algae that may be beneficialto the sulfur compound reducing process. The injected oxygen may be inthe form of air (e.g., dry air with 20.9% oxygen), a gas stream withvarying concentrations of oxygen, substantially pure oxygen or the like.The hydrocarbon and oxygen may be delivered continuously orintermittently, and may be delivered together or separately, e.g.,through the same injectors or through different injectors.

Hydrocarbon-utilizing bacteria may oxide heavy metals through directmetabolism, sequential metabolism, reductive metabolism and/orcometabolism. Furthermore, the hydrocarbons may chemically oxidize orotherwise remediate the metals or metal compounds without the action ofmicroorganisms.

In one embodiment of the invention, remediation of metals may occur bothaerobically and anaerobically. For example, hydrocarbons such as butanemay transform aerobic conditions to anaerobic conditions by initiallyaccelerating the growth of aerobic hydrocarbon-utilizing microorganismsin the presence of oxygen, which produces carbon dioxide and transformsthe aerobic conditions to anaerobic conditions. Under anaerobicconditions, T. Ferrooxidans activity may decrease or terminate, andanaerobic hydrocarbon-utilizing bacteria may flourish. Ultimately, thetransformation from aerobic to anaerobic conditions may prevent orreduce heavy metal migration and curtail T. Ferrooxidans in general. Inaddition, the aerobic cycle may accelerate heavy metal precipitation outof solution, thereby inhibiting the migration of the metals through thesubsurface, or facilitating the collection and removal of these metalsusing ex-situ techniques. Metal contaminants may be remediated bychanging the subsurface microbial ecology of contaminated sites.

Remediation may be conducted either in-situ or ex-situ. In-situequipment may include injection wells for the continuous or periodicdelivery of the hydrocarbon substrate, oxygen and/or nutrients. Forexample, in-situ systems as described in U.S. Pat. Nos. 6,244,346 and6,245,235 may be used to inject the hydrocarbon substrate and,optionally, oxygen to the remediation site.

Ex-situ equipment may include bioreactors, for example, as disclosed inU.S. Pat. Nos. 5,888,396 and 6,051,130, which are capable of treatingair, soil or groundwater waste streams. The ex-situ bioreactor may beused in a batch-type process and/or in a continuous flow process.Ex-situ equipment may also include, for example, butane/air diffusers,precipitation lagoons with metal deposition membrane liners, anaerobicreduction chambers, and aerobic precipitation chambers.

FIG. 1 illustrates a system for in-situ treatment ofarsenic-contaminated groundwater or other types of metal-contaminatedgroundwater in accordance with an embodiment of the present invention.Butane and air injection wells 10 are installed in-situ within a flowpath of metal-contaminated groundwater 12 to create radii of influence14 around the wells 10. Dissolved butane and oxygen thus form a barrieragainst arsenic migration. As the treatment continues, butane-utilizingbacteria produce the requisite enzymes to precipitate arsenic or othermetal contaminants from the groundwater 12 onto aquifer solids. Cleangroundwater 16 then flows toward a recovery well 18, e.g., a drinkingwater well.

FIG. 2 illustrates a system 20 for ex-situ treatment ofmetal-contaminated soil such as arsenic-containing soil in accordancewith another embodiment of the present invention. The system 20 includesa rock crusher 22 where contaminated soil is pretreated or crushed. Thecrushed soil 23 is fed to a slurry bioreactor 24 which includes ahydrocarbon and oxygen supply 25 and diffusers 26. Thehydrocarbon/oxygen supply 25 may comprise, for example, a cylindercontaining a hydrocarbon such as butane and an air compressor, or anyother suitable hydrocarbon/oxygen source. Alternatively, the supply 25may only introduce a hydrocarbon into the bioreactor 24 if anaerobicconditions are desired. The hydrocarbon injected through the diffusers26 stimulates the growth of hydrocarbon-utilizing bacteria in thebioreactor 24, which oxidize or otherwise separate the metal contaminantfrom the soil. In an alternative embodiment, the bioreactor 24 may bereplaced with a washing tank where the metal contaminant is removed fromthe soil without the use of the hydrocarbon.

The clean soil 27 is removed from the bioreactor 24 and themetal-contaminated effluent 28 is pumped 29 to a precipitation lagoon 30where further treatment by hydrocarbon-utilizing bacteria results inmetal precipitation onto a membrane liner 31. The membrane 31 may bemade of any suitable material, such as polyethylene, EPDM rubber,polyurethane or polypropylene. A hydrocarbon and oxygen supply 32 anddiffusers 33 deliver, for example, butane and air to the precipitationlagoon 30. Although two different hydrocarbon supplies 25 and 32 areshown in FIG. 2, a single supply could be used. Clean water 34 is thenpumped 35 from the precipitation lagoon 30 and the metal precipitatesare eliminated, for example, by collection, separation, incineration,disposal and/or stabilization, e.g., with road construction materialssuch as concrete and/or asphalt.

Alternatively, metal-contaminated soil may be treated in heap pilesutilizing leaching techniques. Hydrocarbon and air injection wells maybe installed in the heap pile. Stimulated hydrocarbon-utilizing bacteriamay precipitate the metal while water flushing over the heap collectsthe oxidized metal fraction and creates a solution effluent, which thencan be treated separately, for example, in a precipitation lagoon asdescribed above.

FIG. 3 illustrates a system for treatment of a contaminated site, suchas a heavy metal-contaminated site and/or an acid mine drainage site inaccordance with a further embodiment of the invention. Butane (and/orother hydrocarbon substrates) and oxygen injection wells 40 areinstalled in-situ within a groundwater flow path 42 adjacent to a wasterock area 43 which may be above grade and/or below grade. For example,the waste rock 43, existing above and/or below ground, may result frommetal mining operations and may be a source of AMD or heavy metalcontamination. The wells 40 are installed in-situ and create radii ofinfluence 44 which form a protective curtain or barrier to reduce oreliminate the flow of AMD or heavy metals. The injection wells 40 mayoperate aerobically, for example, by maintaining constant orintermittent air flow and constant or intermittent hydrocarbon flow.Alternately, the injection wells 40 may alternate between periodichydrocarbon injection only and hydrocarbon/air injection to achievealternating anaerobic and aerobic processes. With the transformation ofthe groundwater flow 42 to a substantially anaerobic state, and in thepresence of an alternate electron acceptor such as carbon dioxide ornitrate, the hydrocarbon may serve as an electron donor and carbonsource, thereby halting the AMD process.

FIG. 4 illustrates an ex-situ system 50 for the treatment of lagoons ortanks contaminated with AMD or other heavy metal contaminants inaccordance with an embodiment of the present invention. The ex-situsystem 50 includes a lagoon 52 contaminated with acid mine drainage orother heavy metal contaminants. The contaminated fluid 54 is pumped 56to a precipitation lagoon 58 lined with a membrane 60. A hydrocarbon andoxygen supply 62 and diffusers 64 inject the desired amounts ofhydrocarbon and oxygen at the desired intervals in order to createanaerobic, aerobic or alternate anaerobic and aerobic conditions in theprecipitation lagoon 58. For example, butane and air may be injected inorder to stimulate the growth of aerobic butane-utilizing bacteria whichaccelerate heavy metal precipitation onto the membrane filter 60installed in the precipitation lagoon 58. Clean water 66 is then pumpedfrom the precipitation lagoon 58 while the metal contaminant isdeposited on the membrane filter 60. The membrane 60 may be made of anysuitable material, such as polyethylene, EPDM rubber, polyurethane orpolypropylene.

FIG. 5 illustrates a system 70 for treating spent metal platingsolutions in accordance with another embodiment of the invention. Spentmetal plating solution 72 is pumped 74 to an anaerobic reduction chamber76. A hydrocarbon supply 78 such as a butane cylinder and diffusers 80inject the desired amount of hydrocarbon at the desired intervals intothe reduction chamber 76. The chamber 76 is vented 82 to atmosphere,e.g., by a one-way valve. The solution 84 is then pumped 86 from thereduction chamber 76 to an aerobic precipitation chamber 88. Ahydrocarbon and oxygen supply 90 and diffusers 92 inject the desiredamounts of hydrocarbon and oxygen, such as butane and air, at thedesired intervals into the aerobic precipitation chamber 88. A membraneliner 94 is provided at the bottom of the chamber 88, e.g., on apull-out tray. The chamber 88 is vented 96 to atmosphere. Clean water 98exits the chamber 88.

In accordance with the embodiment shown in FIG. 5, butane-utilizingbacteria or other hydrocarbon-utilizing bacteria in the reductionchamber 76 anaerobically pretreat the metal plating solution 72, e.g.,using metabolic and cometabolic processes. Under anaerobic conditions,hydrocarbon-utilizing bacteria such as butane-utilizing bacteria mayutilize a variety of alternate electron acceptors such as sulfate,nitrate or iron. The electroless plating solution 72 may be pretreatedwith buffers to maintain a pH between 4 and 8. Following treatment inthe reduction chamber 76, the solution 84 undergoes microbial oxidationin the aerobic precipitation chamber 88. The alternate electronacceptors may be added, for example, to the butane/air mix injected intothe precipitation chamber 88. Precipitated metals are deposited on themembrane liner 94 which may be incorporated into pull-out trays forsubsequent removal. A low voltage current may be passed through aportion of the tray assembly to electrolyze and plate portions of thetray liners with the metal constituents to aid and expedite the metalrecovery process.

The following example illustrates the treatment of a spentelectroplating solution, and is not intended to limit the scope of thepresent invention.

EXAMPLE

A 4-liter sample of spent electroplating solution was collected from anindustrial metal-plating facility. Prior to treatment with butane, oneliter of the sample was submitted to a certified analytical laboratoryfor evaluation of the following parameters: pH; total cyanide; and totalmetals. The results of the characterization-sampling event aresummarized in Table 2 below. TABLE 2 Parameter Results pH 3.0 Totalcyanide 0.08 mg/l Antimony <0.1 mg/l Arsenic <0.1 mg/l Beryllium 0.002mg/l Cadmium <0.050 mg/l Chromium 1.34 mg/l Copper 0.16 mg/l Lead <0.04mg/l Mercury <0.0005 mg/l Nickel 765 mg/l Selenium <0.2 mg/l Silver<0.20 mg/l Thallium <0.1 mg/l Zinc 3.10 mg/lmg/l = milligrams per liter

The spent electroplating solution had a pH of 3.0. In addition, theconcentrations of cyanide, beryllium, chromium, copper, nickel and zincwere detected above the laboratory detection limits. Ideally, prior totreatment with butane, the sample should have been adjusted with analkaline buffer to raise the pH. However, the example was designed todemonstrate the principal of metals precipitation under conservativeconditions. Therefore, the pH was not adjusted and since oxygen alonewill partially oxidize metals, air was not pumped into the bioreactorsdesigned for this study.

The study demonstrated metal precipitation under both anaerobic andaerobic conditions. FIG. 6 illustrates an anaerobic bioreactor 100 usedin the study. The reactor 100 included a container 102 having a lid 104,an injection tube 106 and a syringe port 108 to inject butane. A venttube 112 was connected through the lid 104 to a water bath 114. Filterpaper 110 was placed at the bottom of the bioreactor container 102.Three liters of spent electroplating solution underwent butane treatmentin the anaerobic bioreactor 100 for 14 days.

After 14 days, the vent tube 112 and water bath 114 shown in FIG. 6 wereremoved from the bioreactor 100. The resultant aerobic bioreactor 120 isillustrated in FIG. 7. Upon insertion of the vent tube 122, air exchangewas permitted within the vessel 102, although to a limited degree sincebutane is heavier than air and displaces air in a semi-closedenvironment.

During the 28 days of the experiment, butane was injected according tothe schedule shown in Table 3. TABLE 3 Day No. Butane Gas (ml) Condition1 100 Anaerobic 2 200 Anaerobic 3 — Anaerobic 4 100 Anaerobic 5 —Anaerobic 6 400 Anaerobic 7 150 Anaerobic 8 100 Anaerobic 9 150Anaerobic 10 100 Anaerobic 11 100 Anaerobic 12 100 Anaerobic 13 100Anaerobic 14 100 Anaerobic 15 100 Aerobic 16  50 Aerobic 17 150 Aerobic18 150 Aerobic 19 150 Aerobic 20 150 Aerobic 21 — Aerobic 22 150 Aerobic23 150 Aerobic 24 — Aerobic 25 — Aerobic 26 — Aerobic 27 150 Aerobic 28150 Aerobic

Following the 28-day experiment, the spent electroplating solution wasdecanted from the bioreactor. A change in color was immediatelynoticeable. The initial color of the solution was a deep bluish-green.After the 28-day period, the color was light green. The pH of thesolution was tested and found to be the same, i.e., 3.0. A precipitatewas noticeable on the filter paper. The filter paper was submitted to acertified analytical laboratory for metals analyses (only for thosemetals detected above the detection limit during thepre-characterization sampling event). The results are listed in Table 4.TABLE 4 Parameter Results pH 3.0 Total cyanide <0.25 mg Beryllium <0.05mg Chromium 3.5 mg Copper <2.5 mg Nickel 505 mg Zinc 8.5 mg

As demonstrated by the results shown in Table 4, during the relativelyshort experiment the introduction of butane into the spentelectroplating solution caused metals precipitation onto the filterpaper located at the bottom of the bioreactor vessel.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

1. A system for remediating a metal contaminant comprising means fortreating the metal contaminant with alkane-utilizing bacteria in thepresence of an alkane comprising butane, ethane and/or propane.
 2. Thesystem of claim 1, wherein the alkane comprises butane.
 3. The system ofclaim 2, wherein the butane is provided as a butane substrate comprisingbutane as the most prevalent compound of the substrate.
 4. The system ofclaim 2, wherein the butane is provided as a butane substrate comprisingat least about 10 weight percent butane.
 5. The system of claim 2,wherein the butane is provided as a butane substrate comprising at leastabout 50 weight percent butane.
 6. The system of claim 2, wherein thebutane is provided as a butane substrate comprising at least about 90weight percent butane.
 7. The system of claim 2, wherein the butane isprovided as a butane substrate comprising at least about 99 weightpercent n-butane.
 8. The system of claim 1, wherein the alkane comprisesethane.
 9. The system of claim 1, wherein the alkane comprises propane.10. The system of claim 1, wherein the metal contaminant comprises atleast one heavy metal.
 11. The system of claim 10, wherein the at leastone heavy metal comprises arsenic, antimony, beryllium, cadmium,chromium, copper, lead, mercury, iron, manganese, magnesium, radium,nickel, selenium, silver, thallium and/or zinc.
 12. The system of claim1, wherein the metal contaminant comprises arsenic.
 13. The system ofclaim 1, wherein the metal contaminant comprises selenium.
 14. Thesystem of claim 1, wherein the metal contaminant comprises chromium. 15.The system of claim 1, wherein the metal contaminant comprises aradioactive metal.
 16. A remediation system for treating a metalcontaminant comprising: a source of an alkane substrate comprisingbutane, ethane and/or propane; and at least one injector in flowcommunication with the source of the alkane substrate and the metalcontaminant.
 17. The system of claim 16, wherein the alkane comprisesbutane.
 18. The system of claim 17, wherein the butane is provided as abutane substrate comprising butane as the most prevalent compound of thesubstrate on a weight percentage basis.
 19. The system of claim 17,wherein the butane is provided as a butane substrate comprising at leastabout 10 weight percent butane.
 20. The system of claim 17, wherein thebutane is provided as a butane substrate comprising at least about 50weight percent butane.
 21. The system of claim 17, wherein the butane isprovided as a butane substrate comprising at least about 90 weightpercent butane.
 22. The system of claim 17, wherein the butane isprovided as a butane substrate comprising at least about 99 weightpercent n-butane.
 23. The system of claim 16, wherein the alkanecomprises ethane.
 24. The system of claim 16, wherein the alkanecomprises propane.
 25. The system of claim 16, wherein the metalcontaminant comprises at least one heavy metal.
 26. The system of claim25, wherein the at least one heavy metal comprises arsenic, antimony,beryllium, cadmium, chromium, copper, lead, mercury, iron, manganese,magnesium, radium, nickel, selenium, silver, thallium and/or zinc. 27.The system of claim 16, wherein the metal contaminant comprises arsenic.28. The system of claim 16, wherein the metal contaminant comprisesselenium.
 29. The system of claim 16, wherein the metal contaminantcomprises chromium.
 30. The system of claim 16, wherein the metalcontaminant comprises a radioactive metal.
 31. The system of claim 16,further comprising means for supplying oxygen to the metal contaminant.32. The system of claim 31, wherein the oxygen is supplied in the formof air.
 33. The system of claim 31, wherein oxygen is supplied in theform of substantially pure oxygen.
 34. The system of claim 31, whereinthe oxygen is supplied to the metal contaminant continuously.
 35. Thesystem of claim 31, wherein the oxygen is supplied to the metalcontaminant intermittently.